Federal agencies are using Cyber Monday as a warning to consumers about the risk of buying counterfeit merchandise.

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For the third year in a row, Homeland Security investigators seized dozens of websites selling knockoff items in conjunction with what’s believed to be the busiest online shopping day of the year.

Displaying a table full of Patriots, Bruins, and Red Sox memorabilia, all of them counterfeit, special agent Bruce Foucart says it’s become a major enterprise.

“These were found in the flea markets, in people’s homes, you can buy them on Craigslist for example,” said Foucart, estimating copyright infringement costs licensed manufacturers and retailers between $200 billion and $500 billon every year.

After the crackdown, domain names with legitimate-sounding names that once sold illegal gear now direct consumers to federal websites, though the operation spans far beyond sports gear. Watches, designer clothing, and even knockoff versions of normally pricey Beatz-brand headphones have been seized.